With respect to thin steel sheets used for a frame component of an automobile, high strengthening is required in order to achieve safety against collision and improvement of fuel economy. Therefore, it is required to secure press formability while increasing the strength of the steel sheet to 980 MPa class or more. In order to achieve both of high strengthening and securing of formability in high-strength steel sheets of 980 MPa class or more, use of steel utilizing the TRIP effect is known to be effective (refer to Patent Literature 1 for example).
In the Patent Literature 1, a high-strength steel sheet is disclosed which has a main phase of bainite or bainitic ferrite and contains retained austenite (γR) by 3% or more in terms of area ratio. However, with respect to this high-strength steel sheet, the total elongation does not reach 20% at 980 MPa or more of the tensile strength at room temperature, and further improvement of the mechanical property (hereinafter also referred to simply as “property”) is required.
On the other hand, because there is a limit in formability even with a TRIP steel sheet in cold forming, a technology is proposed in which the TRIP effect is further effectively exerted and the elongation is increased by working at 100-400° C. in order to further improve the elongation (refer to Non-patent Literature 1 and Patent Literature 2).
As shown in Table 2 of the Patent Literature 2, by making γR with 1 mass % or more carbon content present in the structure mainly composed of bainitic ferrite, the elongation (total elongation) in the vicinity of 200° C. can be improved to 23% in 1,200 MPa class. However, when press forming is taken into consideration, if local deformation region is utilized particularly in forming in which bulging and deep drawing are main, the strain is localized to cause breakage, and therefore uniform deformation region is often utilized. Accordingly, improvement of only the total elongation including the local elongation is insufficient, and improvement of the uniform elongation is required.
With respect to the uniform elongation, in Patent Literature 3, it is disclosed that the uniform elongation improves by adding Y and REM, however the technology can be applied only for a steel sheet with the tensile strength (TS) of up to 875 MPa as shown in its Table 3. Also, in Patent Literature 4, it is disclosed that the balance of the strength and the uniform elongation improves by a mixed structure of bainitic ferrite-polygonal ferrite-retained austenite, however the technology can also be applied only to a steel sheet with up to 859 MPa TS as shown in its Table 2.
Therefore, development of a technology that could achieve excellent uniform elongation even in a steel sheet of 980 MPa class or above was required.